Cadillac Northstar DIS Theory & Operation
Introduction:
The Cadillac Northstar ignition system on a factory 1993-1999 Northstar engine is composed of an ignition module, four coils, a crankshaft
trigger wheel, two variable reluctance crankshaft position sensors, and a cam sensor used for SFI operation by the PCM. The two
crankshaft position sensors are used to quickly determine crankshaft position for fast engine starting, know as "Fast-Start".
Using two crankshaft sensors allows the ignition module to determine the engine position within 180 degrees of rotation of the crankshaft.
Another benefit of two crankshaft sensors is redundancy; in case one sensor fails, the module can operate off the remaining working
sensors. Since the second sensor is not needed for the module to operate, it will not be used on this conversion to simplify the
overall process. The downfall of this is that the engine will have to crank longer before starting. This is not a problem
for me and is actually desired to build oil pressure. Correction: I was initially wrong about the redundancy of
the crank sensors. If you are running two crank sensors and a cam sensor any one sensor can fail, including one of the crank
sensors and the module will continue to operate. If you operate the module without a cam sensor both crank sensors are needed
to maintain operation.
Crankshaft Trigger Wheel (reluctor ring)
The Northstar crankshaft has a trigger wheel (also called a reluctor ring) cast into the crankshaft between #3 and #4 main bearings.
Two crankshaft position sensors are installed into the side of the engine block (facing front of car in FWD applications) between
cylinders 4 and 6. The trigger wheel has 32 notches, 16 large raised areas, and 16 small raised areas, as seen in Figure 1 and
2 below.
Figure 1: GM drawing of Northstar trigger wheel Image from GM, Cadillac division. |
Figure 2: Cadillac Northstar crank trigger wheel put into CAD Note: Drawing not to scale, for accurate scaled version see design and fabrication page. |
Each notch and small raised area is 1/3 the size of the large raised area. In the GM drawing of the Northstart in Figure 1
you can see that they blacked out the notches between the small raised areas to create 24 even notches and large raised areas.
This is why GM considerers this a 24x trigger wheel. The eight uneven notches are used by the ignition module to determine crankshaft
position to determine which cylinder is next in the firing order.
Northstar PCM & ICM wiring
Note: The firing order above is that of a GM northstar block and must be changed for SBC applications, see
below
Crankshaft Position sensors
The two crankshaft position sensors used in the Northstar setup are variable reluctance sensors separated 27 degrees apart. Each sensor is mounted within a tight tolerance of the trigger wheel (reluctor wheel), 0.05" (1.3mm) on the factory engine.
Camshaft Position sensors
A camshaft position sensor is not needed on HEI ECM conversions because they are bank fire applications this is only needed for SFI
PCM's.
The camshaft position sensor is positioned over a dowel pin located on the exhaust camshaft cam gear. One Off-On-Off signal pulse
is created every camshaft rotation and every two crankshaft rotations.
Coils
The four coils used in the Northstar setup employ a waste spark firing method. Waste spark firing means that two spark plugs are fired at one time thus lowering the number of coils needed and the cost. When a coil fires of the two cylinders fired, one is on the compression stroke and one is on the exhaust stroke.
Ignition Control Module
The Northstar ignition control module is what processes the input from three sensors and the proper signal output to the ECM \ ECU
\ PCM.
It performs the following functions and more:
- It monitors the On-Off pulses produced by the two crankshaft and one camshaft position sensors.
- It creates a 4X and 24X reference signal (4X REF HI and 24X Crank) sent to the PCM for ignition control.
- It creates a camshaft reference signal (CAM HI) sent to the PCM for fuel injection control.
- It provides a ground reference (REF LO, CAM LO) to the PCM.
- It provides a means for the PCM to control spark advance (BYPASS and IGNITION CONTROL) called IGNITION CONTROL MODE.
- It provides a limited means of controlling spark advance without PCM input called MODULE MODE.
- The IC module is not repairable. When a module is replaced the remaining components must be transferred to the new module.
Ignition Synchronization
| Ignition Control Module signal description | |||
| Single | Signal Name | Voltage Range | Description |
| A | Crankshaft position sensor 'A' | +/- 100V AC | Voltage and frequency vary with engine speed |
| B | Crankshaft position sensor 'B' | +/- 100V AC | Voltage and frequency vary with engine speed. Signal lags crankshaft position sensor 'A' by 27 degrees |
| C | Camshaft position sensor | +/- 100V AC | Voltage and frequency vary with engine speed This signal occurs every two crankshaft revolutions |
| D | 4X Ref Hi | 0 - 5V DC | Key on, Engine Off: Signal is high. High to low occurs at first sync, then follows firing order. |
| E | Bypass | 0 - 5V DC | Key on, Engine Off: Signal is low. Low to High occurs with PCM ignition control. |
| F | Ignition control | 0 - 5V DC | PCM sends ignition control signal to ICM. The falling edge signals ICM to turn off coil primary current. |
| G | 24X Crank | 0 - 5V DC | This signal will remain high if the ICM identifies a fault with one of its three position sensorts |
| H | 1/2x Cam Hi | 0 - 5V DC | The PCM uses this signal for sequential (SFI) fuel control |
| I | Ground Ref. | 0V DC | This is the ground reference between the PCM and ICM |
1) The crankshaft reluctor ring has 24 evenly spaced notches plus eight additional notches (shaded) used for synchronization
for a total of 32 notches.
2) As the crankshaft rotates, the notches pass the position sensors and create a voltage pulse signal in the sensor
that is an input for the ignition control module (ICM).
3) Because of the physical location of the 2 crankshaft position sensors, the signal of B lags the signal of A by 27
degrees of crankshaft revolution.
4) To synchronize the ignition, the ICM first counts the number of B pulses between every 2 A pulses. There can be
0, 1, or 2 B pulses between A pulses.
5) When the ICM sees 0 B pulses between A pulses , it starts counting B pulses between A pulses. When the ICM counts
exactly 4, it synchronizes the ignition on the very next A pulse. If the ICM counts over 4 (jumps from 3 to 5), it waits for another
B pulse between A pulse to start counting again.
6) This process allows the ignition to synchronize and fire the first spark plug within 180 degrees (1/2 engine revolution).
7) The camshaft position (CMP) sensor provides the ICM with cylinder #1 firing order information, which the PCM uses
for sequential fuel injection.
8) Using 3 sensors allows the ICM to maintain ignition synchronization even if one of the 3 sensors fails.
Firing Order:
To run the northstar ignition module on a SBC you will connect the wires to the coils as follows.
1'st Gen Northstar Firing order:
1-2-7-3-4-5-6-8
1'st Gen SBC Firing Order:
1-8-4-3-6-5-7-2 looking at the front of the motor counting back: driver side (right) = 1-3-5-7 pass side (left) = 2-4-6-8
Ignition Control Module Pinouts:
I have created a column in the tables below to outline which pins would be used if using the northstar ignition module on a 1227730, 1227727, or pretty much any HEI distributor OBD-I GM car.
| Connector 1 - Crankshaft Sensors | ||
| Pin | Used on HEI ECM | Function |
| A | Yes | Crankshaft sensor A high signal |
| B | Yes | Crankshaft sensor A low signal |
| C | No | Not Used |
| D | No | Not Used |
| E | No | Crankshaft sensor B low signal |
| F | No | Crankshaft sensor B high signal |
| Connector 2 - Power and Tachometer output | ||
| Pin | Used on HEI ECM | Function |
| A | Yes | Ground |
| B | Yes | Tachometer signal output |
| C | Yes | Ignition controlled battery voltage |
| Connector 3 - Cam sensor | ||
| Pin | Used on HEI ECM | Function |
| A | No | Camshaft sensor high output |
| B | No | Camshaft sensor low output |
| C | No | Not Used |
| Connector 4 - PCM \ ECM Output | ||
| Pin | Used on HEI ECM | Function |
| A | No | 24x spark reference signal |
| B | No | 1/2x cam signal |
| C | Yes | 4x fuel control signal |
| D | Yes | Ignition bypass |
| E | Yes | Ignition control |
| F | Yes | Ignition reference low |
Page created: January 2006
Page content last updated: 09-04-2009